1. Field of the Invention
The present invention relates to a droplet ejecting head, a method for driving the same and a droplet ejecting apparatus, and particularly, relates to a droplet ejecting head, a method for driving the same and a droplet ejecting apparatus, for ejecting a fine droplet from a nozzle to thereby record characters or graphics on a recording medium or form a fine pattern or a thin film on a substrate.
2. Description of the Related Art
There is generally known a droplet ejecting method using an electromechanical transducer such as a piezoelectric actuator for generating a pressure wave (acoustic wave) in a pressure generating chamber filled with liquid so as to eject a droplet from a nozzle coupled with the pressure generating chamber due to the pressure wave. Particularly, an ink jet recording apparatus for ejecting ink droplets to thereby record characters or graphics on recording paper has been in widespread use (e.g. JP-B-Sho.53-12138 and JP-A-Hei.10-193587).
FIG. 6 is a view showing an example of a droplet ejecting mechanism (ejector) in a known ink jet recording apparatus as disclosed in these official gazettes. A nozzle 2 for ejecting ink and an ink supply path 5 for introducing ink from an ink tank (not shown) through a common flow path 4 are coupled with a pressure generating chamber 1. In addition, a diaphragm plate 6 is provided in the bottom surface of the pressure generating chamber 1. To eject a droplet, the diaphragm plate 6 is displaced by a piezoelectric actuator 7 provided outside the pressure generating chamber 1 so as to produce a change of volume in the pressure generating chamber 1. Thus, a pressure wave is generated in the pressure generating chamber. Due to this pressure wave, a part of ink charged into the pressure generating chamber 1 is jetted to the outside through the nozzle 2. Thus, the ink flies as a droplet 8. The flying droplet 8 lands on a recording medium such as a sheet of recording paper so as to form a recording dot. Such recording dots are formed repeatedly in accordance with image data. Thus, characters or graphics are recorded on the recording medium.
Further, in recent years, it has been attempted to utilize such a droplet ejecting apparatus for industrial use. For example, major applications include a) to form a wiring pattern or a transistor by ejecting a conductive polymer solution onto a substrate, b) to form an EL display panel by ejecting an organic EL solution onto a substrate, c) to form bumps for electrical mounting by ejecting molten solder onto a substrate, d) to create a three-dimensional object by laminating droplets of UV-curing resin or the like on a substrate and curing the droplets of the resin, and (e) to form an organic thin film by ejecting a solution of organic material (e.g. resist solution) onto a substrate. In such a manner, the droplet ejecting apparatus is being used in a broad variety of fields as well as for image recording, and it is expected that the range of its applications will expand more broadly.
“Reduction of droplet volume” is presently a great technical subject in such a droplet ejecting apparatus. That is, when the droplet ejecting apparatus is used for printing a photographic image or the like, it is important to make recording dots (picture elements) to be formed on a sheet of recording paper as small as possible in order to obtain high image quality with few granularity. To this end, it is necessary to make the apparatus eject very fine droplets. Also when the droplet ejecting apparatus is utilized for industrial applications, it is necessary to eject extremely fine droplets onto a substrate in order to obtain a high-density wiring pattern or a high-resolution EL display panel. The required volume of the fine droplets varies largely in accordance with how to use the droplet ejecting apparatus. For example, when an image is recorded (printed), it is substantially sufficient that fine droplets of 1-2 pl (picoliter) can be ejected. However, in order to form a high-density wiring pattern or a transistor, fine droplets not larger than 0.1 pl have to be ejected. Thus, with the range of applications of a droplet ejecting apparatus being expanded, “reduction of droplet volume” has come into a technical subject more important than ever.
As a driving method for carrying out ejection of a fine droplet in a droplet ejecting head, there is known a driving method in which a pressure generating chamber is once expanded immediately before ejection, and a droplet is ejected in the state where a meniscus in an aperture portion of a nozzle has been retracted toward the pressure generating chamber (JP-A-Sho.55-17589). FIG. 7A shows an example of a driving waveform improved in such a driving method. Incidentally, the relationship between the driving voltage and the behavior of a piezoelectric actuator varies in accordance with the structure of the actuator or the polarization direction thereof. In this specification, assume that the volume of the pressure generating chamber is reduced when the driving voltage is increased, and on the contrary, the volume of the pressure generating chamber is increased when the driving voltage is reduced.
The driving waveform shown in FIG. 7A is constituted by a first voltage change process 51 for expanding the pressure generating chamber and a second voltage change process 52 following the first voltage change process 51 for compressing the pressure generating chamber to thereby eject a droplet.
FIGS. 8A to 8D are views schematically showing the behavior of a meniscus in the nozzle aperture portion when the driving waveform shown in FIG. 7A is applied. A meniscus 9 is flat initially (FIG. 8A). When the pressure generating chamber is expanded immediately before ejection, the meniscus 9 is shaped as shown in FIG. 8B. That is, the central portion of the meniscus 9 is retracted largely toward the pressure generating chamber so that the meniscus 9 is formed into a concave shape. When the pressure generating chamber is compressed by the second voltage change process 52 in the state where the meniscus 9 is formed thus into a concave shape, a thin liquid column 22 is formed in the central portion of the meniscus 9 as shown in FIG. 8C. Next, a droplet 8 is separated from the tip portion of the liquid column 22 so as to be formed (FIG. 8D). The droplet diameter at that time is substantially equal to the thickness of the formed liquid column 22 and smaller than the aperture diameter of the nozzle 2. That is, by use of such a driving method, it is possible to eject a droplet 8 smaller than the nozzle aperture diameter. Incidentally, the driving method in which the meniscus shape immediately before ejection is controlled thus for ejecting a fine droplet will be hereinafter referred to as “meniscus control system” in this specification.
In addition, the present inventor disclosed a driving method capable of stably ejecting a smaller droplet using a driving waveform as shown in FIG. 7B, in JP-A-2000-117969. This driving waveform is constituted by a first voltage change process 51 for retracting a meniscus immediately before ejection, a second voltage change process 52 for compressing a pressure generating chamber to thereby form a liquid column, a third voltage change process 53 for separating a droplet from the tip portion of the liquid column at an early stage, and a voltage change process 54 for suppressing reverberation of a pressure wave surviving after ejection of the droplet. That is, the driving waveform of FIG. 7B includes voltage changes designed to separate a droplet at an early stage and to suppress reverberation. Consequently, a droplet (about 4 pl) smaller in volume than that by use of the driving waveform of FIG. 7A can be ejected stably.
In addition, as a method capable of ejecting a further smaller fine droplet, the inventors disclosed a driving method using proper vibration (natural oscillation) of a piezoelectric actuator in JP-A-2000-218778. This driving waveform has a feature in that a voltage change time t3 of the second voltage change process 52 and a voltage change time t5 of the third voltage change process 53 are set to be not larger than a natural period (natural oscillation period) Ta of a piezoelectric actuator itself respectively. Consequently, the natural vibration of the piezoelectric actuator itself is excited so that high-frequency vibration can be generated in the meniscus. Accordingly, in combination of the high-frequency vibration with the meniscus control system, it is possible to eject a droplet smaller than that in a normal meniscus control system.
In addition, on the basis of the result of making investigations into the ejecting mechanism using the meniscus control system, the inventors disclosed driving waveforms advantageous to ejection of fine droplets in JP-A-2001-63042 and JP-A-2000-146992. In these driving waveforms, a voltage change time t1 of the first voltage change process 51 and a time difference t2 between the completion time of the first voltage change process 51 and the start time of the second voltage change process 52 are set to satisfy specific conditions. Consequently, the phases of particle velocities generated in nodes A, B and C of the driving waveform (see FIG. 7B) are substantially matched to one another so that the particle velocity at the time of applying the second voltage change process 52 can be increased suddenly. As will be described later, when there appears a large change in particle velocity at the time of applying the second voltage change process 52, strong interference of the meniscus occurs in the nozzle central portion so that a thin liquid column is formed. As a result, it is possible to eject a very fine droplet at a high velocity.
However, the droplet volume of the fine droplet that can be actually ejected by such a driving waveform as in the related art has a lower limit at about 1-2 pl. Particularly, it is impossible to eject a fine droplet not larger than 1 pl, which is required for industrial applications of the droplet ejecting apparatus.
In addition, in the droplet ejecting apparatus in the related art, there is another problem that the stability of ejection of a fine droplet is low. That is, it is indeed possible to eject a fine droplet of about 1-2 pl using a driving waveform as shown in FIG. 7B, but it is extremely difficult to carry out uniform ejection from a plurality of ejectors. One of the reasons why there is a variation in ejection state of fine droplets among the ejectors is that a fine droplet ejecting phenomenon in the related art is very sensitive to the nozzle shape or the pressure wave. That is, when a plurality of ejectors are disposed in a head, there is a variation, if slightly, among the ejectors as to the aperture diameters or sectional shapes of their nozzles, or the natural periods of pressure waves generated therein. The fine droplet ejecting method in the related art is very sensitive to such a variation so that there occurs a variation in fine droplet ejection state among the ejectors. Thus, it is difficult to carry out uniform ejection of fine droplets.